The present invention relates to pressure swing adsorption processes, systems and apparatus for the separation of a multi-component teed gas mixture by selectively adsorbing at least one more readily adsorbable component in a bed of adsorbent material.
Gas separations by pressure swing adsorption (PSA) are achieved by coordinated pressure cycling of a bed of adsorbent material which preferentially adsorbs at least one more readily adsorbable component present in a feed gas mixture relative to at least one less readily adsorbable component present in the feed gas mixture. That is, the bed of adsorbent material is contacted with a ready supply of a feed gas mixture. During intervals while the bed of adsorbent material is subjected to the ready supply of feed gas mixture and the bed is at or above a given feed pressure, a supply of gas depleted in the at least one more readily adsorbable component may be withdrawn from the bed. Eventually, the adsorbent material in the bed becomes saturated with the at least one more readily adsorbable component and must be regenerated. At which point, the bed is isolated from the ready supply of feed gas mixture and a gas enriched in the at least one more readily adsorbable component is withdrawn from the bed, regenerating the adsorbent material. In some instances, the bed may be subjected to a feed of depleted gas to facilitate the regeneration process. Once the adsorbent material is sufficiently regenerated, the bed is again subjected to the ready supply of feed gas mixture and depleted gas can once again be withdrawn from the bed once the pressure on the bed is at or above the given feed pressure. This cycle may be performed repeatedly as required. The period of time required to complete one such cycle is referred to as the xe2x80x9ccycle timexe2x80x9d.
The cyclic nature of the basic pressure swing adsorption process has resulted in the development of multibed systems which can provide a continuous stream of depleted gas. By way of example, one widely used system described in Wagner U.S. Pat. No. 3,430,418, herein incorporated by references as if set forth herein in its entirety, employs four adsorbent beds arranged in a parallel flow relationship. Each bed in the four bed system proceeds sequentially through a multistep cycle. Because a depleted gas stream cannot be withdrawn from a given bed continuously, the four beds are arranged so that a depleted gas stream may be withdrawn from at least one of the four beds at all times.
The efficiency of the separation of a gaseous mixture achieved using a given pressure swing adsorption system depends on various parameters, including the feed pressure, the regeneration pressure, the cycle time, the pressure gradient established across the bed, the type of adsorbent material as well as its size and shape, the dimensions of the adsorption beds, the amount of dead volume in the system, the composition of the gaseous mixture to be separated, the uniformity of flow distribution, the system temperature and the temperature gradient established within said bed. Variations in these parameters can influence the cost and productivity of a given system.
Every pressure swing adsorption system contains dead volume. For purposes of explanation of this dead volume, a conventional two bed pressure swing adsorption system is depicted in FIG. 1. The conventional system comprises two identical beds 10 and 50 of adsorbent material 5. Each of the identical beds 10 and 60 have: a feed gas inlet valve 15 and 55, respectively; a depleted gas outlet valve 20 and 60, respectively; and an enriched gas outlet valve 25 and 65, respectively. The depleted gas outlet valves 20 and 60 are in fluid communication with bed outlet conduits 21 and 61, respectively, and depleted gas conduit 70. The conventional system further comprises a feed prime mover 30 and an exhaust prime mover 40. The feed prime mover 30 intakes a feed gas mixture from the atmosphere or a storage container (not shown) and exhausts the feed gas mixture through feed gas inlet conduit 75 which is in fluid communication with the feed gas inlet valves 15 and 55. The feed gas inlet valves 15 and 55 are also in fluid communication with bed inlet/outlet conduits 16 and 56, respectively. The enriched gas outlet valves 25 and 65 are in fluid communication with the bed inlet/outlet conduits 16 and 56, respectively; and an enriched gas conduit 80. The enriched gas conduit 80 is also in fluid communication with the exhaust prime mover 40.
It should be recognized that the dead volume of a pressure swing adsorption system includes (a) an xe2x80x9cinlet void volumexe2x80x9d which is the volume that is in fluid communication with the inlet end of the bed of adsorbent material and (b) an xe2x80x9coutlet void volumexe2x80x9d which is the volume that is in fluid communication with the outlet end of the bed of adsorbent material. It should be understood that, for the purposes of this disclosure, the sum of the xe2x80x9cinlet void volumexe2x80x9d and the xe2x80x9coutlet void volumexe2x80x9d for a given pressure swing adsorption system is the xe2x80x9ctotal dead volumexe2x80x9d for the given pressure swing adsorption system. It should also be recognized that the bed of adsorbent material will itself contain a xe2x80x9cbed void volumexe2x80x9d which includes the void spaces between and around the individual particles of adsorbent material or, in the case of structured adsorbents, the spaces not occupied by particles of adsorbent material. It should be understood that for the purposes of this disclosure, the xe2x80x9ctotal dead volumexe2x80x9d does not include the xe2x80x9cbed void volumexe2x80x9d.
For example, in FIG. 1, the xe2x80x9cinlet void volumexe2x80x9d 12 for the bed of adsorbent material 10 is indicated using a dashed line. Likewise, the xe2x80x9coutlet void volumexe2x80x9d 11 for the bed of adsorbent material 10 is indicated using a dashed line. That is, the xe2x80x9cinlet void volumexe2x80x9d 12 is the volume which is in communication with the inlet end of the bed of adsorbent material 10. In the pressure swing adsorption system depicted in FIG. 1, it is therefore the sum of the volume of (a) the inlet/outlet conduit 16 between the bed 10 and the bed side of the feed gas inlet valve 15 and the enriched gas outlet valve 25 and (b) the free inlet space 13, the free inlet space may contain a flow distribution system and/or confinement means for supporting the adsorbent material within the bed. Similarly, the xe2x80x9coutlet void volumexe2x80x9d 11 is the volume which is in communication with the outlet end of the bed of adsorbent material 10. In the pressure swing adsorption system depicted in FIG. 1, it is therefore the sum of the volume of (a) the outlet conduit 21 between the bed 10 and the bed side of the depleted gas outlet valve 20 and (b) the free outlet space 14, for example space required by a confinement means for retaining the adsorbent material within the bed and reducing the potential for fluidizing the adsorbent material.
Every pressure swing adsorption system contains some dead volume. Notwithstanding, the benefits of reducing the size of the dead volume are readily understood by those skilled in the art. Such benefits include improved recovery and productivity. Recognizing the benefits of reducing the total dead volume of a pressure swing adsorption system, however, is quite distinct from recognizing how to effect a reduction in the total dead volume.
To avoid an early breakthrough of an impurity through the bed, conventional adsorption systems incorporate a flow distributor in fluid communication with the inlet of the bed. The purpose of the flow distributor is to distribute the flow of feed gas uniformly across the entire bed cross-section to avoid inefficiencies caused by such early breakthroughs of impurities. Conventional flow distributors, however, introduce dead volume into the system. As noted above, such dead volume tends to negatively influence the efficiency of the gas separation.
Many conventional adsorption systems use a single pump or other type of conventional prime mover to transfer a feed gas mixture into the bed during one part of the adsorption cycle and withdraw an enriched gas from the bed during another part of the cycle. Because the direction of flow through conventional prime movers cannot be quickly reversed, a complex valving system is employed to change the direction of flow of gas relative to the bed. Such a system is described in, for example, U.S. Pat. No. 6,156,101. The increase in complexity associated with the use of such valving systems provides additional opportunity for system failures.
It is well known that, at least in theory, a decrease in the cycle time for a given adsorption system should decrease the adsorbent requirement and facilitate a reduction in the overall size and weight of the system apparatus. In practice, however, decreases in cycle time introduce a plethora of operational challenges. For example, decreases in cycle times invariably necessitate an increased frequency of valve switching, which may reduce system reliability. Valves require a certain amount of time to transition from one position to another. Hence, as the cycle time becomes increasingly short (i.e., approaches the transition time for the valves), the system efficiency will actually be seen to decrease with further decreases in the cycle time. The pressure difference between the bed inlet and the bed outlet is another operational challenge to the implementation of decreased cycle times. That is, this pressure difference increases as the cycle time decreases. The significance of this increased pressure difference can be alleviated to a degree by employing a bed having a relatively short bed depth. For systems which exhibit a relatively short bed depth, the pressure gradient (pressure difference divided by bed depth) will be relatively large, but the pressure difference will be relatively small. In theory, operation in this high pressure gradient regime is beneficial; however, in conventional systems, the total dead volume tends to be large relative to the bed volume (i.e., the volume physically occupied by adsorbent material) and the shortness of the bed depth presents a significant flow distribution challenge. As a result of the challenges associated with flow distribution and void volume, discussed above, the potential benefits of operating in this high-pressure gradient regime have not been fully realized.
While pressure swing adsorption processes typically operate at pressures above atmospheric pressure, some may operate at pressures below atmospheric pressure during all or part of the adsorption cycle and may be described as vacuum swing adsorption (VSA) or vacuum-pressure swing adsorption (VPSA) processes. For the purposes of the instant specification, the term xe2x80x9cpressure swing adsorptionxe2x80x9d (PSA) is used generically to describe all types of cyclic adsorption processes including vacuum swing adsorption and vacuum-pressure swing adsorption processes.
The xe2x80x9cvolume of the bedxe2x80x9d, xe2x80x9cbed volumexe2x80x9d or Vbed, as these terms are used herein and in the appended claims, is the volume physically occupied by the adsorbent material. For adsorbent beds comprised of individual porous particles, the volume of the bed includes the interstitial void space between adjacent particles, the volume of void space within the individual particles, and the volume occupied by the solid portion of the adsorbent particles.
The xe2x80x9cbed depthxe2x80x9d, L, as that term is used herein and in the appended claims, is the shortest distance through the bed from the surface where feed gas enters the bedxe2x80x94the xe2x80x9cinlet surfacexe2x80x9dxe2x80x94to the surface where the depleted gas exits the bedxe2x80x94the xe2x80x9coutlet surfacexe2x80x9d.
The xe2x80x9cequivalent diameterxe2x80x9d, as that term is used herein and in the appended claims, is the diameter of a sphere having a volume equivalent to the volume of a given particle.
The xe2x80x9caverage equivalent diameterxe2x80x9d, {overscore (dp)}, as that term is used herein and in the appended claims, is according to the following equation,       dp    _    =            ∑              all        ⁢                  xe2x80x83                ⁢        i              ⁢                  x        i            ⁢              dp        i            
where xi is the weight fraction of particles with equivalent diameter dpi.
xe2x80x9cStandard liters per minutexe2x80x9d, as that term is used herein and in the appended claims, is the volume of gas at a temperature of 25xc2x0 C. and a pressure of 1 atmosphere.
xe2x80x9cPrime moverxe2x80x9d, as that term is used herein and in the appended claims, means any pump, compressor, blower or similar device suitable for facilitating the transfer of a fluid, particularly a vapor or gas, from one place to another.
In an embodiment of the present invention, a pressure swing adsorption process for the separation of a multi-component feed gas mixture is provided which involves selectively adsorbing at least one more readily adsorbable component of the multi-component feed gas mixture in a bed of adsorbent material by a process including (a) pressurizing the bed; (b) passing the feed gas mixture through the bed from an inlet surface of the bed to an outlet surface of the bed, wherein the at least one more readily adsorbable component is preferentially adsorbed by the adsorbent material, and withdrawing a depleted gas depleted in the at least one more readily adsorbable component exiting from the outlet surface of the bed; (c) depressurizing the bed by withdrawing an enriched gas enriched in the at least one more readily adsorbable component from the bed; (d) repeating (a)-(c) in a cyclic manner.
In one aspect of this embodiment of the present invention, the bed has a bed depth, L, and a bed volume, Vbed; wherein the bed has an aspect ratio according to equation (1)
Aspect ratio=L3/Vbedxe2x80x83xe2x80x83(1)
wherein the aspect ratio for the bed is less than 10, less than 5, less than 0.25, or less than 0.125; and, wherein the mean pressure gradient in the bed in (b) exceeds 0.035 psi/inch, exceeds 0.05 psi/inch, exceeds 0.5 psi/inch, or exceeds 1 psi/inch.
In another aspect of this embodiment of the present invention, the feed gas mixture is air and the depleted gas contains oxygen at a concentration of at least 70 mol %, at least 80 mol %, or at least 90 mol %.
In another aspect of this embodiment of the present invention, the depleted gas contains hydrogen at a concentration of at least 80 mol %, at least 95 mol %, or at least 99 mol %.
In another aspect of this embodiment of the present invention, the production rate for the depleted gas is between 0.5 and 10 standard liters per minute, between 0.5 and 5 standard liters per minute, or between 1 and 3 standard liters per minute.
In another aspect of this embodiment of the present invention, each repeat of (a) through (c) defines a cycle with a cycle time of 30 seconds or less, less than 15 seconds, less than 6 seconds, or less than 1 second.
In another aspect of this embodiment of the present invention, the bed contains adsorbent particles with an average equivalent diameter of less than 1.0 mm, less than 0.5 mm, or less than 0.1 mm.
In another aspect of this embodiment of the present invention, the bed may contain structured adsorbent. For example, the bed may contain structured adsorbents selected from the group of monoliths, laminates, gauzes, and other adsorbent supports.
In another aspect of this embodiment of the present invention, the bed exhibits a Void factor according to equation (2) of less than 0.2, less than 0.1, or less than 0.05,                               Void          ⁢                      xe2x80x83                    ⁢          factor                =                  Vvoid                      Vvoid            +            Vbed                                              (        2        )            
wherein Vvoid is the total dead volume in the pressure swing adsorption system and Vbed is the bed volume.
In another embodiment of the present invention, a pressure swing adsorption process for the separation of a multi-component feed gas mixture is provided which involves selectively adsorbing at least one more readily adsorbable component of the multi-component feed gas mixture in a bed of adsorbent material having an inlet surface and an outlet surface by a process including (a) pressurizing the bed; (b) passing the feed gas mixture through the bed, wherein the at least one more readily adsorbable component is preferentially adsorbed by the adsorbent material, and withdrawing a depleted gas depleted in the at least one more readily adsorbed component exiting from the bed; (c) halting the withdrawal of the depleted gas from the bed and halting the feed of the feed gas mixture to the bed; (d) depressurizing the bed by withdrawing an enriched gas enriched in the at least one more readily adsorbable component from the bed; (e) feeding a portion of the depleted gas into the bed to facilitate the extraction of the at least one more readily adsorbable component from the adsorbent material, (f) repeating operations (a)-(e) in a cyclic manner; wherein the at least one more readily adsorbable component is preferentially adsorbed by the adsorbent material at the feed pressure, wherein the bed has a bed depth, L, and a bed volume, Vbed; wherein the bed has an aspect ratio according to equation (1)
Aspect ratio=L3/Vbedxe2x80x83xe2x80x83(1)
wherein the aspect ratio for the bed is less than 10, less than 5, less than 0.25, or less than 0.125; and, wherein the mean pressure gradient in the bed in (b) exceeds 0.035 psi/inch, exceeds 0.05 psi/inch, exceeds 0.5 psi/inch, or exceeds 1 psi/inch.
In an aspect of this embodiment of the present invention, each repeat of (a) through (e) defines a cycle with a cycle time of 30 seconds or less, less than 15 seconds, less than 6 seconds, or less than 1 second.
In another embodiment of the present invention, a pressure swing adsorption system for the separation of a multi-component feed gas mixture is provided which involves selectively adsorbing at least one more readily adsorbable component of the multi-component feed gas mixture in a bed of adsorbent material including: (a) at least one bed of the adsorbent material, wherein the adsorbent material more strongly adsorbs the at least one more readily adsorbable component and wherein the at least one bed has at least one inlet surface and at least one outlet surface: (b) at least one prime mover supplying the multi-component feed gas mixture to the pressure swing adsorption system; and (c) an array of valves in fluid communication with the at least one inlet; wherein the valves cycle from an open to a closed position in 1 second or less, less than 0.1 second, or less than 0.01 second.
In an aspect of this embodiment of the present invention, the at least one bed exhibits a bed depth, L, and a bed volume, Vbed; wherein the at least one bed has an aspect ratio according to equation (1)
Aspect ratio=L3/Vbedxe2x80x83xe2x80x83(1)
wherein the aspect ratio for the at least one bed is less than 10, less than 5, less than 0.25, or less than 0.125.
In another aspect of this embodiment of the present invention, the at least one bed exhibits a Void factor according to equation (2) of less than 0.2, less than 0.1, or less than 0.05,                               Void          ⁢                      xe2x80x83                    ⁢          factor                =                  Vvoid                      Vvoid            +            Vbed                                              (        2        )            
wherein Vvoid is the total dead volume in the pressure swing adsorption system and Vbed is the bed volume.
In another aspect of this embodiment of the present invention, the array of valves contains at least 4 individual valves, at least 20 valves, or at least 100 valves.
In another aspect of this embodiment of the present invention, the valves in the array of valves are selected from piezoelectric valves, shape memory alloy valves, electrostatic valves, bimetallic valves, thermopneumatic valves and electromagnetic valves.
In another embodiment of the present invention, a pressure swing adsorption system for the separation of a multi-component feed gas mixture is provided which involves selectively adsorbing at least one more readily adsorbable component of the multi-component feed gas mixture in a bed of adsorbent material including: (a) at least one bed of the adsorbent material, wherein the adsorbent material more strongly adsorbs the at least one more readily adsorbable component and wherein the at least one bed has at least one inlet surface and at least one outlet surface; and (b) at least one array of prime movers in fluid communication with the at least one inlet surface; wherein flow through the at least one array of prime movers can be started or stopped in less than 2 seconds, less than 0.5 seconds, or less than 0.1 seconds.
In an aspect of this embodiment of the present invention, the at least one bed has a bed depth, L, and a bed volume, Vbed; wherein the at least one bed has an aspect ratio according to equation (1)
Aspect ratio=L3/Vbedxe2x80x83xe2x80x83(1)
wherein the aspect ratio for the at least one bed is less than 10, less than 5, less than 0.25, or less than 0.125.
In another aspect of this embodiment of the present invention, the at least one bed has a Void factor according to equation (2) of less than 0.2, less than 0.1, or less than 0.05,                               Void          ⁢                      xe2x80x83                    ⁢          factor                =                  Vvoid                      Vvoid            +            Vbed                                              (        2        )            
wherein Vvoid is the total dead volume in the pressure swing adsorption system and Vbed is the bed volume.
In another aspect of this embodiment of the present invention, the pressure swing adsorption system may further include at least one array of exhaust prime movers in fluid communication with the at least one outlet surface of the bed.
In another aspect of this embodiment of the present invention, the at least one array of prime movers contains: an array of feed prime movers in fluid communication with the at least one inlet surface of the at least one bed, wherein the array of feed prime movers operates to feed the feed gas mixture into the at least one bed; and, an array of exhaust prime movers in fluid communication with the at least one inlet surface of the at least one bed, wherein the array of exhaust prime movers operates to withdraw an enriched gas from the at least one bed.
In another aspect of this embodiment of the present invention, the at least one array of prime movers operates to transfer the feed gas mixture into the at least one bed during a feed gas feeding step and operates to withdraw an enriched gas from the bed during a regeneration step.
In another aspect of this embodiment of the present invention, each array of prime movers contains at least 4 individual prime movers, at least 20 individual prime movers, or at least 100 individual prime movers.
In another aspect of this embodiment of the present invention, the individual prime movers in the at least one array of prime movers should be capable of initiating or stopping the flow of gas therethrough in 2 seconds or less, less than 0.5 seconds, or less than 0.1 seconds and include, but are by no means limited to, piezoelectric pumps, thermopneumatic pumps, electrostatic pumps, ultrasonic pumps, electro-osmosis pumps, electrohydrodynamic pumps, electromagnetic pumps, rotary pumps, shape memory alloy pumps, bimetallic pumps, diaphragm pumps, rotary vane pumps, scroll pumps, solenoid pumps, stepper-motor actuated pumps, piston pumps, linear pumps. In a particular aspect of this embodiment, the individual prime movers in the at least one array of prime movers include microelectromechanical (MEM) pumps selected from MEM piezoelectric pumps, MEM thermopneumatic pumps, MEM electrostatic pumps, MEM electromagnetic pumps, MEM ultrasonic pumps, MEM electro-osmosis pumps, MEM diaphragm pumps and MEM electrohydrodynamic pumps.
In another embodiment of the present invention, an apparatus for the pressure swing adsorption of a multi-component feed gas mixture by selectively adsorbing at least one more readily adsorbable component on an adsorbent material, is provided including: (a) a bed of adsorbent material with at least one inlet surface and at least one outlet surface; wherein the adsorbent material more strongly adsorbs the at least one more readily adsorbable component; and (b) an array of valves in fluid communication with the at least one inlet surface; wherein the valves cycle from an open to a closed position in 1 second or less, less than 0.1 seconds, or less than 0.01 seconds.
In an aspect of this embodiment of the present invention, the at least one bed exhibits a bed depth, L, and a bed volume, Vbed; wherein the at least one bed has an aspect ratio according to equation (1)
Aspect ratio=L3/Vbedxe2x80x83xe2x80x83(1)
wherein the aspect ratio for the at least one bed is less than 10, less than 5, less than 0.25, or less than 0.125.
In another aspect of this embodiment of the present invention, the at least one bed exhibits a Void factor according to equation (2) of less than 0.2, less than 0.1, or less than 0.05,                               Void          ⁢                      xe2x80x83                    ⁢          factor                =                  Vvoid                      Vvoid            +            Vbed                                              (        2        )            
wherein Vvoid is the total dead volume in the pressure swing adsorption system and Vbed is the bed volume.
In another aspect of this embodiment of the present invention, the at least one bed exhibits a geometry selected from the group consisting of a cylinder with a circular cross section, a cylinder with a non-circular cross section, a rectangular parallelepiped, or the annular region between two coaxial cylinders.
In another aspect of this embodiment of the present invention, the apparatus is designed and sized to be carried by an individual.
In another aspect of this embodiment of the present invention, the apparatus is designed to provide oxygen for medical purposes.
In another aspect of this embodiment of the present invention, the apparatus is designed to provide hydrogen for use in a fuel cell.
In another embodiment of the present invention, an apparatus for the pressure swing adsorption of a multi-component feed gas mixture by selectively adsorbing at least one more readily adsorbable component on an adsorbent material, is provided including: (a) a bed of adsorbent material with at least one inlet surface and at least one outlet surface; wherein the adsorbent material more strongly adsorbs the at least one more readily adsorbable component; and (b) at least one array of prime movers in fluid communication with the at least one inlet surface; wherein flow through the at least one array of prime movers can be started or stopped in 2 seconds or less, less than 0.5 seconds, or less than 0.1 seconds.
In an aspect of this embodiment of the present invention, the at least one bed exhibits a bed depth, L, and a bed volume, Vbed; wherein the at least one bed has an aspect ratio according to equation (1)
Aspect ratio=L3/Vbedxe2x80x83xe2x80x83(1)
wherein the aspect ratio for the at least one bed is less than 10, less than 5, less than 0.25, or less than 0.125.
In another aspect of this embodiment of the present invention, the at least one bed exhibits a Void factor according to equation (2) of less than 0.2, less than 0.1, or less than 0.05,                               Void          ⁢                      xe2x80x83                    ⁢          factor                =                  Vvoid                      Vvoid            +            Vbed                                              (        2        )            
wherein Vvoid is the total dead volume in the pressure swing adsorption system and Vbed is the bed volume.
In another aspect of this embodiment of the present invention, the apparatus is designed and sized to be carried by an individual.
In another aspect of this embodiment of the present invention, the apparatus is designed to provide oxygen for medical purposes.
In another aspect of this embodiment of the present invention, the apparatus is designed to provide hydrogen for use in a fuel cell.